The term xanthophylls means carotenoid pigments having an oxygen-containing group such as a hydroxyl group, a keto group or an epoxy group. Carotenoids are synthesized by the isoprenoid biosynthetic process which is used in common halfway with steroids and other terpenoids with mevalonic acid as a starting material. C15 farnesyl pyrophosphate (FPP) resulting from isoprene basic biosynthetic pathway is condensed with C5 isopentenyl pyrophosphate (IPP) to give C20 geranylgeranyl pyrophosphate (GGPP). Two molecules of GGPP are condensed to synthesize a colorless phytoene as an initial carotenoid. The phytoene is converted into phytofluene, .zeta.-carotene, neurosporene and then lycopene by a series of desaturation reactions, and lycopene is in turn converted into .beta.-carotene by the cyclization reaction. It is believed that a variety of xanthophylls are synthesized by introducing a hydroxyl group or a keto group into the .beta.-carotene (See Britton, G., "Biosynthesis of Carotenoids"; Plant Pigments, Goodwin, T. W. ed., London, Academic Press, 1988, pp. 133-182).
The present inventors have recently made it possible to clone a carotenoid biosynthesis gene cluster from a epiphytic non-photosynthetic bacterium Erwinia uredovora in Escherichia coli with an index of the yellow tone of the bacterium, a variety of combinations of the genes being expressed in microorganisms such as Escherichia coli to produce phytoene, lycopene, .beta.-carotene, and zeaxanthin which is a derivative of .beta.-carotene into which hydroxyl groups have been introduced (See FIG. 10; Misawa, N., Nakagawa, M., Kobayashi, K., Yamano, S., Izawa, Y., Nakamura, K., Harashima, K.; "Elucidation of the Erwinia uredovora Carotenoid biosynthetic Pathway by Functional Analysis of Gene Products Expressed in Escherichia coli", J. Bacteriol., 172, p.6704-6712, 1990; Misawa, N., Yamano, S., Ikenaga, H., "Production of .beta.-carotene in Zymomonas mobilis and Agrobacterium tumefaciencs by Introduction of the Biosynthesis Genes from Erwinia uredovora", Appl. environ. Microbiol., 57, p. 1847-1849, 1991; and Japanese Patent Application No. 58786/1991 (Japanese Patent Application No. 53255/1990): "DNA Strands useful for the Synthesis of Carotenoids").
On the other hand, astaxanthin, a red xanthophyll, is a typical animal carotenoid which occurs particularly in a wide variety of marine animals including red fishes such as a sea bream and a salmon, and crustaceans such as a crab and a lobster. In general, animals cannot biosynthesize carotenoids, so that it is necessary for them to ingest carotenoids synthesized by microorganisms or plants from their environments. Thus, astaxanthin hitherto has been used widely for strengthening the color of cultured fishes and shellfishes such as sea bream, salmon, lobster and the like. Moreover, astaxanthin has attracted attention not only as a coloring matter in foods but also as an anti-oxidant for removing active oxygen generated in bodies, which causes carcinoma (see Takao Matsuno ed., "Physiological Functions and Bioactivities of Carotenoids in Animals", Kagaku to Seibutsu, 28, p. 219-227, 1990). As the sources of astaxanthin, there have been known crustaceans such as a krill in the Antarctic Ocean, cultured products of a yeast Phaffia, cultured products of a green alga Haematococcus, and products obtained by the organic synthetic methods. However, when crustaceans such as krill in the Antarctic Ocean or the like are used, requires laborious work and much expense is required for the isolation of astaxantin from contaminants such as lipids and like during harvesting and extraction of krill. Moreover, in the case of the cultured products from the yeast Phaffia, a great deal of expense is incurred for the gathering and extraction of astaxanthin, because yeast has rigid cell walls and produces astaxanthin in a low yield. Also, in the case of the cultured product of the green alga Haematococcus, not only a location for collecting sunlight or an investment of a culturing apparatus for supplying an artificial light is required in order to supply light which is essential to the synthesis of astaxantin, but also it is difficult to separate astaxanthin from fatty acid esters as by-products or chlorophylls present in the cultured products. For these reasons, astaxanthin produced from biological sources presently is inferior to that obtained by organic synthetic methods due to cost. Organic synthetic methods however, produce by-products. Thus, in context of its use as a feed for fishes and shellfishes and an additive to foods, the products obtained by these organic synthetic methods are unacceptable due to the consumer's preference for natural products. Accordingly, it is desired to supply an inexpensive astaxanthin that is safe and produced from biological sources and thus present a good image to consumers, and to develop a process for producing astaxanthin.